Catalytic processes for the conversion of hydrocarbons are well known and extensively used. In many of these processes, the catalyst consists of particles that are transported between two or more catalyst-containing vessels. The reason why the catalyst is transported varies depending on the process. For example, the catalyst may be transported from one reaction vessel into another reaction vessel in order to take advantage of different reaction conditions in the two vessels in order to improve product yields. In another example, the catalyst may first be transported from a reaction vessel into a regeneration vessel in order to rejuvenate the catalyst, and after rejuvenation, the catalyst may be transported back to the reaction vessel.
The vessels between which the catalyst is transported are not necessarily adjacent, and indeed it is common that the outlet of the source vessel, that is the vessel from which the catalyst is transported, may be a significant distance horizontally and vertically from the inlet of the destination vessel, that is the vessel to which the catalyst is transported. An inexpensive and common method of transferring catalyst over significant vertical and horizontal distances is by pneumatic conveying through a conduit. Pneumatic conveying is well known to those skilled in the art of transporting particles. Pneumatic conveying is described at pages 5-46 to 5-48 in Perry's Chemical Engineers' Handbook, Sixth Edition, ed. by Don W. Green, McGraw-Hill ed., McGraw-Hill Book Company, New York, 1984.
One of the characteristics of pneumatic conveying is that because of the pressure difference across the conduit between the source and destination vessels, the destination vessel must operate at a pressure that is lower than that of the source vessel. In many catalytic processes, however, for process reasons the destination vessel operates at a higher pressure than that of the source vessel. In such processes, therefore, pneumatic conveying by itself is not sufficient to transfer catalyst from the source vessel to the destination vessel, and a supplemental method that is capable of transferring catalyst from low pressure to high pressure must be used. A common and inexpensive method of transferring catalyst through a significant increase in pressure is by lock hopper. Lock hoppers are well known to those skilled in the art of transporting particles. The use of lock hoppers in combination with pneumatic conveying in a two-step method is also well known. First, a lock hopper transfers catalyst from the low pressure source vessel to a pneumatic conveying system that is at a higher pressure than that of the destination vessel. Second, the pneumatic conveying system transfers the catalyst to the destination zone, which is at a higher pressure than that of the source vessel, in spite of the pressure drop associated with pneumatic conveying.
One of the problems associated with combining a lock hopper and a pneumatic conveying system is that the lock hopper and the pneumatic conveying system can and often do transfer catalyst independently and at different rates. For example, typically a lock hopper transfers catalyst in batches whereas a pneumatic conveying system transfers catalyst continuously. Accordingly, a surge zone is necessary between the lock hopper and the pneumatic conveying system in order to maintain a volume of catalyst to balance the transitory differences in the flow that may occur during intermittent transport of catalyst from the source vessel to the destination vessel. The level of catalyst in the surge zone has been used in a method of controlling the rate of pneumatic conveyance of the catalyst through a conduit to the destination vessel. Typically, this method comprises providing a controller with a desired value of the level of catalyst in the surge zone, measuring with a device the actual value of the level, comparing the desired and actual values, and finally changing the rate of conveyance until the actual and desired values of the level are substantially equal.
Control methods like the ones just described suffer from large and rapid fluctuations in the pressures in the lock hopper, the surge zone, the destination vessel, or all three. If the catalyst enters the surge zone intermittently and in relatively large batches at the same time that catalyst is transported from the surge zone through a pneumatic conduit continuously, the level in the surge zone undergoes a large and rapid change each time a batch of catalyst is added to the surge zone. As a result of the action of the surge zone level controller, such changes from one value of the actual level to another value of the actual level produce large and rapid changes in the transport rate. This, in turn causes large and rapid pressure fluctuations, because in pneumatic conveying the pressure difference across the conduit between the source and destination vessels varies depending on the transport rate of catalyst through the conduit. For example, the pressure difference across the conduit when gas is flowing at its design rate and no catalyst is flowing may be only 1-5 in. H.sub.2 O, but the pressure difference when gas and catalyst are both flowing at their design rates may be 150-250 in. H.sub.2 O. In those processes where catalyst is entering the surge zone by gravity flow at the same time that catalyst is being transported out of the surge zone through the conduit, a surge of 150-250 in. H.sub.2 O in the pressure of the surge zone can make part or all of the catalyst transport system unstable. Because this situation is unacceptable, use of larger and/or extra vessels and higher rates for making up and venting gases from the process in order to attempt to control the pressure fluctuations have been employed. Because these corrective measures are expensive, methods are thus sought for interrelating or integrating the transfer rates of the lock hopper with that of the pneumatic conveying system.